Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens composed of five optical elements which is built in an image pickup device mounted in an increasingly compact and low-profile smartphone, mobile phone, PDA (Personal Digital Assistant), a game console, an information terminal such as a PC, or a home appliance with a camera function.
In the present invention, whether or not an optical element is a lens depends on whether or not it has refractive power on an optical axis. An optical element having refractive power on the optical axis is a lens. An optical element which does not have a lens function can contribute to improvement of aberrations in the peripheral area of an image without changing the overall focal length. In terms of lens surface shape, a convex surface or a concave surface means that the paraxial portion of the surface (the portion near the optical axis) is convex or concave. A “pole-change point” on an aspheric surface means a point on the aspheric surface at which a tangential plane intersects the optical axis perpendicularly.
Description of the Related Art
In recent years, there has been a general tendency that many information terminals have a camera function. Also, home appliances with a camera have been introduced into the market. For example, a user who is away from home can see in real time what is going on at home, through the camera mounted in a home appliance by telecommunication between the home appliance and his/her smartphone. It is thought that products which enhance consumer convenience by adding a camera function to an information terminal or home appliance will be increasingly developed in the future. The camera mounted in such products is required not only to provide high resolution to cope with an increase in the number of pixels but also to be compact and low-profile and offer high brightness and a wide field of view.
However, in order to provide a low-profile imaging lens with a wide field of view and high brightness, the following problem has to be addressed: it is difficult to correct aberrations in the peripheral area of an image and deliver high optical performance throughout the image. Even though the use of glass material for the lenses may address the above problem to a certain degree, glass material is not suitable for mass production and thus it is difficult to achieve low-cost mass production.
Conventionally, for example, the imaging lenses described in JP-A-2010-271541 (Patent Document 1) and JP-A-2010-026434 (Patent Document 2) are known as imaging lenses intended to offer a wide field of view and deliver high performance.
Patent Document 1 discloses an imaging lens which includes, in order from an object side to an image side, a first lens with positive refractive power as a biconvex lens, a second lens with negative refractive power having a concave surface on the image side, a third lens with positive refractive power as a meniscus lens having a convex surface on the image side, and a fourth lens with negative refractive power as a double-sided aspheric lens having a concave surface near an optical axis on the image side.
Patent Document 2 discloses a compact imaging lens intended to correct various aberrations properly, which includes, in order from an object side, a first positive lens, a second positive lens, a third negative lens, a fourth positive lens and a fifth negative lens.
The imaging lens described in Patent Document 1 is relatively low-profile in which the total track length is about 5.4 mm and the ratio of the total track length to the diagonal length of the effective imaging plane of the image sensor is about 0.9. However, since it is composed of four elements, it cannot correct aberrations properly. Also, although it offers a relatively wide field of view, its F-value is about 2.8, which is not sufficient to meet the recent demand for high-brightness imaging lenses.
The imaging lens described in Patent Document 2, composed of five elements, has a total track length of about 7.8 mm and the ratio of the total track length to the diagonal length of the effective imaging plane of the image sensor is about 1.1, which is disadvantageous in making the imaging lens more low-profile. The imaging lens composed of five elements corrects various aberrations properly and offers high brightness with an F-value of about 2.0 to about 2.5. However, its maximum field of view is about 62 degrees and if an attempt is made to widen the field of view, a problem will arise with correction of aberrations in the peripheral area of an image.
As mentioned above, in the conventional art, it is difficult to provide a low-profile high-resolution imaging lens with a wide field of view and high brightness.